1. Field of the Invention
The present invention relates to an apparatus which eliminates noise from an imaging device employed in digital cameras or cellular phones.
2. Description of the Related Art
Imaging devices such as CCD or CMOS sensors, which are employed in digital cameras or cellular phones, are affected by dark current that flows through the imaging elements or pixels even in the absence of incident light, thereby generating certain fixed pattern noise (dark current noise).
An apparatus for eliminating such dark current noise is disclosed in Patent Document 1. This apparatus first performs orthogonal transformation and quantization to compress dark current noise components, i.e., the image captured by an imaging device with the shutter closed, and then stores the resulting components in a memory device. When an image is actually captured, the dark current noise components decoded by inverse orthogonal transformation are subtracted from this image signal, thereby eliminating the dark current noise.
Orthogonal transformation and quantization, which is disclosed in Patent Document 1, as a method for compressing dark current noise, employs a technique that is well known in the art of image compression. This technique makes use of the typical nature of images that processing an image signal by orthogonal transformation causes most information to be given by lower frequency components. Thus, cutting off high frequency component information by quantization does not have many effects on the quality of the resulting image.
Patent Document 1: Japanese Patent Application Laid-Open No. Hei 11-298762.
(First Problem)
However, dark current noise components include many high frequency components and thus cutting off the high frequency component information would cause the dark current noise components to be improperly decoded. Accordingly, when the dark current noise components are compressed by orthogonal transformation and quantization, the high frequency component information cannot be cut off and thus compressibility cannot be improved.
(Second Problem)
Dark current noise components vary significantly depending on the temperature and the exposure time of an imaging device at the time of capturing an image as well as on the gain by which an image signal is multiplied when acquired from the imaging device as an analog signal. Accordingly, the magnitude of a pre-captured dark current noise component differs from the magnitude of a dark current noise component contained in an image signal acquired under actual capturing conditions. The dark current noise eliminator disclosed in Patent Document 1 was adapted to perform compression and decompression but could not properly eliminate the dark current noise components from the image signal because the pre-stored dark current noise components were subtracted, as they were, from the image signal.
(Third Problem)
Any signal delivered from an imaging device such as CCD or CMOS sensors, which are employed in digital cameras or cellular phones, includes the following types of noise in addition to an image signal. These are fixed pattern noise caused by dark current components, smear noise generated by intense incident light, fixed defect noise arising from a CCD defect caused during its manufacture, and random noise resulting from thermal fluctuations of the device. It is thus necessary to eliminate these noise components from signals delivered by the imaging device. In Japanese Patent Application Laid-Open No. 2004-172925, an apparatus is disclosed which first eliminates smear noise from a signal acquired by an imaging device and then removes fixed pattern noise caused by dark current.
To eliminate the smear noise using a circuit, it is necessary to estimate the amount of light incident upon the imaging device in order to estimate the magnitude of the smear noise. In this case, the amount of incident light is to be estimated based on a signal delivered from the imaging device. Accordingly, when the accuracy of estimating the smear noise is taken into consideration, no noise should be included in the signal that is used for the estimation.
However, as described above, a signal delivered from the imaging device includes various types of noise other than the smear noise. Among them is the fixed pattern noise that is caused by dark current. Since the dark current has a high dependence on temperature, an increase in the temperature of the imaging device by 7 degrees Celsius will double the magnitude of the fixed pattern noise. The fixed pattern noise also increases in proportion to the exposure time. As such, the signal delivered from the imaging device has a fixed pattern noise that greatly varies depending on the temperature and the exposure time of the imaging device. Accordingly, the device described in Japanese Patent Application Laid-Open No. 2004-172925 have a problem in that its estimation of the smear noise was greatly affected by the fixed pattern noise, thereby reducing the accuracy with which the smear noise can be eliminated.